A relationship exists between each subscriber and its Home-Public Land Mobile Network (H-PLMN). If communications for a subscriber are handled over another PLMN, this PLMN is referred to as the Visited-PLMN (V-PLMN). A PLMN has a geographical area allocated to it. This geographical area may be referred to as a PLMN area. For example, a PLMN may be limited by the borders of a country. There may be more than one PLMN per country. Using other words, the PLMN area is the geographical area in which a PLMN provides communication services according to the specifications to users of UEs. A group of PLMN areas accessible by UEs may be described as a system area. PLMNs shall provide a location registration function with the main purpose of providing continuity of service to UEs over the whole system area.
A UE may roam (nationally or internationally) and access services in the area authorized by the entitlement of its subscription. Roaming may be described as a service whereby a UE of a given PLMN (e.g. the H-PLMN) is able to obtain service from another PLMN (e.g. the V-PLMN). The other PLMN may be in a different or the same country as the given PLMN.
A location update procedure allows a UE to inform the cellular network, whenever it moves from one location area to the next, e.g. from a H-PLMN to a V-PLMN, from one V-PLMN to another V-PLMN. The location update procedure may be a Location Area Update (LAU) procedure, a Tracking Area Update (TAU) procedure or a Routing Area Update (RAU) procedure. LAU is used in Second Generation (2G) (e.g. Global System for Mobile communications (GSM)), TAU is used in Third Generation (3G) (e.g. Universal Terrestrial Radio Access Network (UTRAN)) and RAU is used in 3G (e.g. Evolved-UTRAN (E-UTRAN)).
As part of a UE registration at a Serving GPRS Support Node/Mobility Management Entity (SGSN/MME), the SGSN/MME sends an Update Location Request message to the Home Subscriber Server (HSS). In the response message from the HSS to the SGSN/MME, e.g. an Update Location Answer message, the HSS may include information about Radio Access Technologies (RAT) restrictions that applies to the UE, i.e. by which additional RATs the UE is granted access. The additional RATs are in addition to the access in which the UE is currently about to be served. The SGSN/MME may use this information to prevent attempts to perform access change to a RAT which is not allowed for the UE
A limitation is that the information about RAT restrictions is signaled common to all Public Land Mobile Network IDentities (PLMN IDs)/operators. The resulting effect is that it is not possible for the HSS to apply differentiated access restrictions per operator, and the receiving SGSN/MME will apply the same restriction regardless of the PLMN ID of the potential target access.
Consider a roaming scenario where a UE from a H-PLMN PLMN-X becomes served in E-UTRAN by the operator of PLMN-1. The HSS provides an access restriction to E-UTRAN only to match roaming agreements which are valid for PLMN-1. The UE is at the same time:                1) allowed, by subscription, to use UTRAN access provided by the operator of PLMN-2, but        2) not allowed to use E-UTRAN access provided by the operator of PLMN-2.        
If the operator of PLMN-1 has a cooperation with the operator of PLMN-2 to ensure good E-UTRAN area coverage, it may cause the operator of PLMN-1 to attempt to move the UE to be served by E-UTRAN at PLMN-2 when radio conditions are in favor to do so. However, since the UE is not allowed to access E-UTRAN at PLMN-2, the attempt to move the UE to be served by E-UTRAN at PLMN-2 will fail.
The HSS includes information about applicable access restrictions per PLMN ID of roaming agreements. This enables the SGSN/MME in the V-PLMN to serve the UE and apply access restrictions matching roaming agreements for the operator of the UE. The V-PLMN would as a result not attempt to move the UE to an access which is not allowed as per subscription.
When the UE roams into a V-PLMN, the HSS sends an Update Location Answer to the SGSN/MME in the V-PLMN which includes access restriction information in the form of a list of pairs (PLMN ID, RAT restriction). The receiving SGSN/MME uses the information to prevent attempts to move the UE to a location/access matching the entries in the list of restrictions.
However, adding such qualifying information cause an issue to configuration of the HSS and to signaling. The reason is that the list of restrictions would basically be a list of all RATs and PLMN IDs on a global scale. Even if it would be possible to limit the list of pairs (PLMN ID, RAT restriction) to entries applicable to the visited country only and potentially adjacent accesses in neighboring countries, the solution would still be inconvenient. The information may potentially be confidential since it will reveal roaming agreements. Signaling of the large amount of data is not feasible from a size and processing point of view. It would require HSS to be configured with roaming relations between V-PLMNs on a global scale.